????????
      +---------
PXZPX | GMZHMTZW
       -PXZPX
        =====
        PXJWXT
       -PTWGYY
        ======
          RPMPZ
         -NLMQU
          =====
           NUHUW
          -NLMQU
           =====
           ?????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium
missing: V